In recent years, a mounting density of electronic parts has been increased in accordance with the progress of electronic apparatuses, and new types of the mounting method are being employed, such as a method of mounting a semiconductor package having almost the same size as that of a semiconductor chip, called chip scale package or chip size package (hereinafter, simply referred to as “CSP”), or mounting a bare chip, etc.
One of the most important characteristics for a mounted board having mounted thereon various electronic parts including a semiconductor element is interconnection reliability. Among them, interconnection reliability against thermal fatigue is directly responsible for a reliability of an apparatus using a mounted board and is therefore a very important issue.
One of the causes of lowering the interconnection reliability is a thermal stress caused by using various materials with different coefficients of thermal expansion. Since a semiconductor element has a coefficient of thermal expansion as small as about 4 ppm/° C., whereas a wiring board on which electronic parts are mounted has a coefficient of thermal expansion as large as 15 ppm/° C. or more, a thermal strain is caused upon a thermal impact, which in turn generates a thermal stress.
A substrate having mounted thereon a semiconductor package having a lead frame such as QFP and SOP, etc. has maintained its reliability by absorbing a thermal stress at a portion of the lead frame.
However, in the bare chip mounting, there is employed a method of connecting an electrode of a semiconductor chip to a wiring board pad of a wiring board using a solder ball, or a method of connecting by preparing a small projection called bump and using a conductive paste. Therefore, a thermal stress is concentrated on this connecting portion, thereby lowering the interconnection reliability. It has been known that introducing an underfill resin between the semiconductor element and the wiring board is effective for dispersing the thermal stress. However, the number of steps for mounting is increased, thereby elevating the cost. On the other hand, there is a method of connecting an electrode of a semiconductor element to a wiring pad of a wiring board using conventional wire bonding, however, in this method, the board must be coated with a resin for encapsulation for protecting the wire, thus increasing the number of steps for mounting.
CSP can be mounted together with other electronic parts, and various structures have been proposed as shown in Table 1 appearing at page 5 of “Future of CSP (fine pitch BGA) put into practical use”, the article of Surface Mounting Technology, March, 1997, published by Nikkan Kogyo Shimbun Ltd. Especially, a system in which a tape and a carrier substrate are used with a wiring board called interposer is being put into practical use. This includes systems shown in the above table that Tessera, Inc. and Texas Instruments Inc. are developing. In these systems, a semiconductor device is mounted through a wiring board called interposer, and hence excellent interconnection reliability is exhibited as reported in Shingaku Technical Report CPM96-121, ICD96-160 (1996-12) “Development of Tape BGA size CSP”, and Sharp Technical Journal, No. 66 (1996-12) “Development of Chip Size Package”.
Between the semiconductor chip of CSP and the wiring board called interposer, an adhesive film is preferably used which lowers the thermal stress caused by the difference in coefficients of thermal expansion between the semiconductor device and the wiring board. Accordingly, the adhesive film is required to have moisture resistance and durability at high temperatures. Further, from the viewpoint of facilitating control of the production process, the adhesive film is desired to be of a film type.
For these adhesive film, in addition to an effect for alleviating thermal stress, heat resistance and moisture resistance, it is required that adhesive should not be oozed out to an electrode portion equipped on a semiconductor chip to output an electric signal, for manufacturing process. In addition, in case of using a wiring board with through-hole, it is requisite that an adhesive should not be oozed out from the through-holes. This is because oozing of the adhesive causes failure in connection of the electrodes, and oozing of the adhesive from the through-holes spoils a metal mold, causing a failure in connection. In addition, since a void between a circuit and an adhesive tends to lower heat resistance and moisture resistance, there should not be a void left between the circuit on a wiring board and the adhesive.
An adhesive of a film type is used in flexible printed circuit boards, and those comprised mainly of an acrylonitrile-butadiene rubber are frequently used.
As printed circuit board materials with an improved moisture resistance, Japanese Provisional Patent Publication No. 243180/1985 discloses an adhesive comprising an acrylic resin, an epoxy resin, polyisocyanate and an inorganic filler, and Japanese Provisional Patent Publication No. 138680/1986 discloses an adhesive comprising an acrylic resin, an epoxy resin, a compound having a urethane bond in the molecule and having primary amine at both terminals thereof and an inorganic filler.
However, these adhesives have disadvantages in that the adhesive force is significantly lowered after being treated at a high temperature for a long time and that a resistance to electrolytic corrosion is insufficient. The adhesives suffer significant deterioration especially in the moisture resistance test under severe conditions in, e.g., a pressure cooker test (PCT) treatment, which is used for the reliability evaluation of semiconductor relating parts.
When the semiconductor chip is mounted on a wiring board, using the adhesive as a printed circuit board relating material, the adhesive film cannot be used since the difference in coefficients of thermal expansion between the semiconductor chip and the interposer is large, thereby easily generating a crack during reflow. In addition, the adhesive film cannot be used, since it suffers significant deterioration when it is subjected to moisture resistance test under severe conditions, e.g., in the temperature cycle test or PCT treatment.
On the other hand, as an adhesive whose adhesiveness is less deteriorated even after being treated at a high temperature for a long period of time, there has been disclosed an adhesive obtained by mixing a reactive acrylic rubber or an acrylonitrile-butadine rubber with an epoxy resin. As for the reactive rubber type adhesive, there is disclosed an adhesive composition comprising an acrylic elastomer containing carboxyl groups and hydroxyl groups or epoxy groups, an alkyl phenol, an epoxy resin and imidazolium trimellitate, as disclosed in Japanese Provisional Patent Publication No. Hei 3-181580, and this is used in a field where a copper film is bonded on a support film of a flexible printed wiring board.
The present inventors have found that, as described in Japanese Provisional Patent Publication No. 2000-154361, by reducing the elastic modulus of the adhesive film at around room temperature, the thermal stress caused in the heating-cooling cycle due to the difference in coeffficient of thermal expansion between the semiconductor chip and the wiring board can be lowered, so that no crack is caused during reflow and no damage is observed after temperature cycle test, thus giving the adhesive film excellent in heat resistance.
However, when the reactive rubber is used, an adhesive of the film type or an adhesive parts using the same tends to change its property on storage in a time dependent-manner, due to a reactivity of the rubber, leading to a loss of fluidity and adhesiveness. Therefore, these films and adhesive parts are required to be stored in a refrigerated or frozen state, or to be used in a short time after it is purchased, leaving a problem as to handling unsolved.
In order to repress a reactivity of a rubber in a reactive rubber type adhesive and to improve a storing property thereof, a capsule type latent curing agent is used in some cases. However, an adhesive film to be used for CSP to which the present invention is objected has a problem such that the above-mentioned capsule type latent curing agent can not be used since the capsule is melt when it goes through several times of heat-treatments when it is laminated on a substrate such as polyimide and a semiconductor chip, prior to carrying out the final connection and adhesion-curing between the semiconductor chip and the substrate for mounting.
However, in the future, when requirements for heat resistance and reflow-resistance are set more strictly, it is necessary to give the adhesive higher levels of heat resistance and moisture resistance by elevating a peeling strength and an elasticity at a higher temperature. In addition, if oozing of the adhesive is considerable, it causes a failure in connectivity, such as failure in connection of electrodes, spoiling of a metal mold, etc., and this problem has to be solved. Further, if storing property is not sufficient, there arises a problem relating to a handling or a problem that flexibility in production is hindered, for example, an adhesive film has to be stored in a refrigerated or frozen state, or it has to be used in a short time after its purchase.
On the other hand, for a method for using an adhesive film for bonding a semiconductor chip and an outer connection part, or bonding semiconductor chips themselves, there are a punch and pressing method and a wafer back surface attaching and press-bonding method.
In the punch and pressing method, a film-press-bonding machine having both a film die-cutting function and a heat-pressing function is used. First, an adhesive film in a sheet form or a reel form is cut by a metal mold in a fixed size, and this is tentatively press-bonded onto a fixed position of an outer connection part. Subsequently, the adhesive film is heat-pressed and bonded using a pressing means. Further, a semiconductor chip is positioned on the adhesive film, and heat-pressed for bonding the outer connection part and the semiconductor chip.
In the wafer back surface attaching and press-bonding method, an adhesive film is attached on a back surface of a wafer onto which semiconductor chips are formed, by means of e.g., heat and pressure laminating method, and then, a dicing tape is laminated thereon. The wafer and the adhesive film are cut as a whole. Subsequently, the dicing tape is peeled off to give a semiconductor with an adhesive film, and this is heat-pressed for bonding to an outer connection part with wiring or another semiconductor chip.
In any method for press-bonding, in case of a face-up structure where a back surface of a chip is faced to the outer connection part, pressuring for bonding is carried out from a chip surface by using a pressing means, it is required that the chips should be pressed by a small pressure to prevent the chip from being broken. Especially in a recent year, in accordance with a trend for thinner and more laminated chips, it is required to carry out press-bonding by a smaller pressure and a lower temperature than ever.
Incidentally, the outer connection part has a structure in which a wiring layer is formed on a base comprising a film substrate such as polyimide, etc. and a rigid substrate such as BT resin, etc. It is categorized in a structure where a wiring layer is faced to a semiconductor chip or to outer connecting terminals, or a structure where wiring layers are provided on both surfaces of the substrate. In a circuit-in structure where the wiring layer is provided on the surface facing the semiconductor, there exists unevenness of about 5 to 20 μm due to a patterning of the wiring layer on a surface of the outer connection part to which the adhesive film is presse-bonded.
However, as to a conventional adhesive film, e.g., the adhesive film disclosed in Japanese Provisional Patent Publication No.2000-256628, filling property toward the unevenness is good under a pressure of 0.5 to 3.0 Mpa, however, when the pressure fells below 0.5 Mpa, filling of the adhesive film can not be fully performed, leaving voids on an adhesive interface. Such voids on the interface lowers a reliability including heat resistance and moisture resistance, therefore, the pressure below 0.5 Mpa has not been employed for bonding conventionally.
Generation of the voids depends on fluidity of the adhesive film, so it is possible to prevent a generation of voids by making melt viscosity small at a pressing temperature, however, when the melt viscosity is too small, excessive adhesive composition is oozed out from the peripheral of the adhesive film when the chip is pressed, causing a bonding failure at a lead terminal part. In addition, in case of using the wafer back surface attaching and press-bonding method, excessive oozing is caused, and a surface of a wafer or a laminated device is spoiled. Moreover, as to workability in a pressing process, there is a problem that the adhesive film adheres to a metal mold and a conveyer in the punch and pressing method, and in the wafer back surface attaching and press-bonding method, there is a problem that peeling a dicing tape is difficult. Therefore, the conventional adhesive films cannot achieve both press-bonding property and workability when it is pressed with a small pressure.
An object of the present invention is to provide an adhesive composition that has heat resistance and moisture resistance required for mounting a semiconductor chip whose coefficient of thermal expansion is largely different on a substrate for mounting a semiconductor, and further has an excellent oozing-resistance and circuit-filling property, as well as good storing property, a process for producing the same, an adhesive film using the same, a substrate for mounting a semiconductor and a semiconductor device.
Another object of the present invention is to provide an adhesive film for connecting semiconductor chips and an outer connection part with wiring which supports the chips, being able to perform bonding by heat-pressing with a pressure of 0.01 to 0.5 MPa, and being excellent in adhesiveness and workability upon press-bonding.